The temperature of Heaven can be rather accurately computed.
Our authority is Isaiah 30:26, "Moreover, the light of the Moon
shall be as the light of the Sun and the light of the Sun shall
be sevenfold, as the light of seven days." Thus Heaven receives
from the Moon as much radiation as we do from the Sun, and in
addition 7*7 (49) times as much as the Earth does from the Sun,
or 50 times in all. The light we receive from the Moon is one
1/10,000 of the light we receive from the Sun, so we can ignore
that... The radiation falling on Heaven will heat it to the
point where the heat lost by radiation is just equal to the heat
received by radiation, i.e., Heaven loses 50 times as much heat
as the Earth by radiation. Using the Stefan-Boltzmann law for
radiation, (H/E)^4 = 50, where E is the absolute temperature
of the earth (-300K), gives H as 798K (525°C).
The exact temperature of Hell cannot be computed... [However]
Revelations 21:8 says "But the fearful, and unbelieving...
shall have their part in the lake which burneth with fire and brimstone."
A lake of molten brimstone means that its
temperature must be at or below the boiling point [of
Sulphur], 444.6°C.
We have, then, that Heaven, at 525°C is hotter than Hell at
445°C.
-- From "Applied Optics" vol. 11, A14, 1972